001/*
002 * Licensed to the Apache Software Foundation (ASF) under one or more
003 * contributor license agreements.  See the NOTICE file distributed with
004 * this work for additional information regarding copyright ownership.
005 * The ASF licenses this file to You under the Apache License, Version 2.0
006 * (the "License"); you may not use this file except in compliance with
007 * the License.  You may obtain a copy of the License at
008 *
009 *      http://www.apache.org/licenses/LICENSE-2.0
010 *
011 * Unless required by applicable law or agreed to in writing, software
012 * distributed under the License is distributed on an "AS IS" BASIS,
013 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
014 * See the License for the specific language governing permissions and
015 * limitations under the License.
016 */
017package org.apache.commons.lang3.concurrent;
018
019import java.util.concurrent.atomic.AtomicReference;
020
021/**
022 * A specialized {@link ConcurrentInitializer} implementation which is similar
023 * to {@link AtomicInitializer}, but ensures that the {@link #initialize()}
024 * method is called only once.
025 *
026 * <p>
027 * As {@link AtomicInitializer} this class is based on atomic variables, so it
028 * can create an object under concurrent access without synchronization.
029 * However, it implements an additional check to guarantee that the
030 * {@link #initialize()} method which actually creates the object cannot be
031 * called multiple times.
032 * </p>
033 * <p>
034 * Because of this additional check this implementation is slightly less
035 * efficient than {@link AtomicInitializer}, but if the object creation in the
036 * {@code initialize()} method is expensive or if multiple invocations of
037 * {@code initialize()} are problematic, it is the better alternative.
038 * </p>
039 * <p>
040 * From its semantics this class has the same properties as
041 * {@link LazyInitializer}. It is a &quot;save&quot; implementation of the lazy
042 * initializer pattern. Comparing both classes in terms of efficiency is
043 * difficult because which one is faster depends on multiple factors. Because
044 * {@link AtomicSafeInitializer} does not use synchronization at all it probably
045 * outruns {@link LazyInitializer}, at least under low or moderate concurrent
046 * access. Developers should run their own benchmarks on the expected target
047 * platform to decide which implementation is suitable for their specific use
048 * case.
049 * </p>
050 *
051 * @since 3.0
052 * @param <T> the type of the object managed by this initializer class
053 */
054public abstract class AtomicSafeInitializer<T> implements
055        ConcurrentInitializer<T> {
056    /** A guard which ensures that initialize() is called only once. */
057    private final AtomicReference<AtomicSafeInitializer<T>> factory =
058            new AtomicReference<>();
059
060    /** Holds the reference to the managed object. */
061    private final AtomicReference<T> reference = new AtomicReference<>();
062
063    /**
064     * Gets (and initialize, if not initialized yet) the required object
065     *
066     * @return lazily initialized object
067     * @throws ConcurrentException if the initialization of the object causes an
068     * exception
069     */
070    @Override
071    public final T get() throws ConcurrentException {
072        T result;
073
074        while ((result = reference.get()) == null) {
075            if (factory.compareAndSet(null, this)) {
076                reference.set(initialize());
077            }
078        }
079
080        return result;
081    }
082
083    /**
084     * Creates and initializes the object managed by this
085     * {@link AtomicInitializer}. This method is called by {@link #get()} when
086     * the managed object is not available yet. An implementation can focus on
087     * the creation of the object. No synchronization is needed, as this is
088     * already handled by {@code get()}. This method is guaranteed to be called
089     * only once.
090     *
091     * @return the managed data object
092     * @throws ConcurrentException if an error occurs during object creation
093     */
094    protected abstract T initialize() throws ConcurrentException;
095}